The present invention generally relates to an improved apparatus and method for calibrating an image-capturing device that is connected to a document feeder. More particularly, it relates to an apparatus and method for calibrating a scanner which positions a calibration strip within the document feeder into an optical path of a scanner head assembly while the assembly is ready to scan a document fed by the document feeder.
Image-capturing peripherals such as scanners have become increasingly useful, affordable and common devices for homes and businesses. These devices are useful for capturing and storing images such as text, graphic or pictorial images contained on documents. Various types of scanners include flatbed, drum and handheld scanners. With a flatbed scanner, one of the most common types of scanners, a document to be scanned is typically placed onto a transparent glass platen of the scanner, where a scanner head assembly (also referred to as a carriage assembly) moves underneath the document to capture the image contained on the document. The image in digital form is often transmitted to a connected computer, though it may instead be stored within the scanner, or transmitted directly to another peripheral such as a printer or facsimile (fax) machine.
To scan a quantity of documents, a document feeder (such as an automatic document feeder or ADF) may be attached to the scanner to feed documents over the scanner head assembly, where the images on the documents are captured as they pass over the scanner head assembly. Often, a scanner and document feeder are integrated with a printer or fax machine to form a multi-function printer. Alternatively, the scanner may digitally send image information to the printer.
FIG. 1 is a cross-sectional view of a typical flatbed scanner combined with a document feeder. A scanner, indicated generally at 10, includes a head assembly 12 having a scanning lamp for producing a light for illuminating a document through a glass platen 14, and may also contain mirrors and a lens to direct and focus the reflected light. The head assembly 12 includes a photodetector, such as a charge-coupled device (CCD) containing an array of pixels, each of which are configured to detect the reflected light and convert it into a signal for processing by another peripheral or by a connected computer (not shown). The head assembly 12 travels longitudinally along one or more rails 16 of the scanner 10, and is driven by a pulley and one or more rollers (not shown).
A document feeder 20, which may be attached to the scanner 10 via hinges (not shown), feeds the paper into a scanning position along a generally U-shaped paper path 22 (more clearly shown in FIGS. 2 and 3), substantially surrounding a guiding mechanism such as a typically hollow cylindrical guide 50. The document feeder 20 uses a feeding mechanism having a series of rollers, including a pick-up roller 24, pairs of feed rollers 26, 28, 30, and a pair of delivery rollers 34 to feed the paper through the paper path 22. A transparent guide strip 36 of polyester, such as MYLAR, guides the paper along a bottom portion of the paper path 22. A section 38 of the guide strip 36 allows the head assembly 12, when in a scan position 40 (as shown in FIGS. 1 and 2) to capture images on paper, because the paper is within an optical path (field of view) of the head assembly.
Before scanning one or more documents, a scanner is typically calibrated for photo response non-uniformity (PRNU) calibration, among other things. FIG. 3 shows a simplified representation of a typical scanner/document feeder with the head assembly 12 in a predetermined “home” (calibration) position 42. To calibrate the scanner 10, a stationary calibration strip 44 is attached to the scanner over the home position 42 of the head assembly 12. The strip is positioned away from the paper path 22 and is thereby protected from dust from the paper being scanned. The head assembly 12 scans the stationary, preferably white calibration strip 40 to calibrate the scanner 10 in a manner known in the art.
When scanning a document fed by the document feeder 20, and referring to FIG. 2, the head assembly 12 has to move from the home position 42 to the scan position 40 so that a portion of the paper being scanned is in the optical path or field of view of the head assembly 12 through the portion 38 of the guide strip 36. The distance between the home position 42 and the scan position 40 is typically about 13 mm, but of course this distance may vary.
To produce a high quality scan, calibration ideally should be done for every scan page. However, for high speed document feeding, it is almost impossible to calibrate every scan page because of the mass and inertia of the head assembly 12. Because additional time and power are required to activate the head assembly 12 for every ADF-fed scan job, the scan performance is gradually degraded, due to wear on the head assembly 12, and its mechanism for movement. The redundant quick and short jacking motion of the head assembly 12 can detrimentally impact the scan quality of the scanner 10.
Because about 80% of overall scan jobs are fed from the document feeder for a typical MFP, a significant improvement in reliability and scan quality of the scanner, as well as the print quality of connected printers, would result if the head assembly remained stationary during ADF-fed scanning jobs.